If you're looking to develop for only WM5 devices (actually WM2003 as well) then go for
VB .NET. It's what all the cool kids are using these days. Going forward with WM5, you can pretty safely target the .NET Compact Framework 2.0 as we are now building that into Apache ROMs and otherwise it's freely downloadable.
You will need to get your hands on Visual Studio 2005 and the WM5 SDK. Then you'll just need to read and read and read some more. There are books on the subject at amazon or your local bookseller. Probably the same info is available online if you get your google on.
I speak as a non-developer, so any more detailed questions will have to be tackled by someone else.
Best of luck!